Teratological effect of yeast grown on n-paraffin was discussed on the data with mice reported to the Ministry of Health and Welfare. In the report1), high incidence of skeletal malformation of fetuses of the mice fed a diet containing 28% of the yeast continuously for 4 generations was described among other findings. On the other hand, multi-generation feeding experiments with breeding hens2-8) on the yeast revealed rather high reproducibility but no data were obtained suggesting teratological effect of the yeast.<br>Low calcium and high phosphorus contents in the yeast tended to result in unbalance of dietary Ca and P levels if special care is not taken in formulting the diet. Depressed appetite, retarded growth and delayed maturity observed in the multi-generation experiment with breeding hens were confirmed to be mainly due to unbalance of dietary Ca and P levels9), 10). Since low Ca level was suspected in the yeast diet fed to mice in the report1) mentioned above, the experiment was repeated on the diets of the same composition as in the report.<br>Four experimental diets were prepared, two of which, i.e., Diet C and Diet Y, had the same composition as the control and yeast diet, respectively, in the report. In Diet C-Ca, dietary Ca level in Diet C was decreased to the level in Diet Y, and in Diet Y+Ca, the level in Diet Y was increased to the level in Diet C. Four lots each of two female and one male adult mice were allotted to each of 4 diets. Another 4 males each were also allotted to each of 4 diets. After 3 weeks, the males were sacrificed to determine ash content in femour and tibia. At 3 weeks after parturition, baby mice were weaned and the females were sacrificed for the determination of the bone ash. Twenty one-week-old male chicks each were fed one of the 4 diets for 6 days and sacrificed for the determination of ash in tibia and toe.<br>In Diets Y and C-Ca, Ca:P ratio was over 1:2, and feeding the diets to the chicks and male mice resulted in significantly lower bone ash content than those fed either Diets C or Y+Ca. Bone ash content of the females on Diet Y was significantly lower than those on the other 3 diets, while that on Diet C-Ca was similar to those on Diets C or Y+Ca. Body weights of the weanlings on unbalanced diet, i.e. Diete Y and C-Ca, were smaller than those on balanced diets, though the difference were not significant statistically.<br>It was discussed that main cause of skeletal malformation of fetuses observed in the report may be due to unbalanced levels of dietary Ca and P.